Carvedilol, (±)-1-(Carbazol-4-yloxy)-3-[[2-(o-methoxyphenoxy)ethyl]amino]-2-propanol, is a nonselective β-adrenergic blocker with α1-blocking activity. Carvedilol is a racemic mixture having the following structural formula:

Carvedilol is the active ingredient in COREG®, which is indicated for the treatment of congestive heart failure and for the management of hypertension. Since carvedilol is a multiple-action drug, its beta-blocking activity affects the response to certain nerve impulses in parts of the body. As a result, beta-blockers decrease the heart's need for blood and oxygen by reducing its workload. Carvedilol is also known to be a vasodilator resulting primarily from alpha-adrenoceptor blockade. The multiple actions of carvedilol are responsible for the antihypertensive efficacy of the drug and for its effectiveness in managing congestive heart failure.
U.S. Pat. No. 4,503,067 (“'067 patent”) discloses a class of carbazolyl-(4)-oxypropanolamine compounds, including carvedilol. See '067 patent, col. 1, 1. 15 to col. 2, 1. 3. The '067 patent also discloses the conversion of the compounds to their pharmacologically acceptable salts, by reacting the compound with “an equivalent amount of an inorganic or organic acid,” such as phosphoric acid. See id. at col. 4, 11. 23-29.
U.S. publication No. 2005/0240027 (“'027 publication”) and U.S. publication No. 2005/0277689 (“'689 publication”) each disclose that carvedilol has “relatively low solubility” (<1 μg/mL) in alkaline media, and that its solubility increases with decreasing pH, up to about 100 μg/mL. See '027 publication, p. 1, 7; '689 publication, p. 1, 7. These publications also disclose solid and crystalline forms of carvedilol salts, as well as solvates thereof. See, e.g., '027 publication, p. 3, 51; '689 publication, p. 5, 169.
The discovery of new salt forms of carvedilol is needed in order to have greater aqueous solubility and also greater chemical stability.
Solid state physical properties of a pharmaceutical compound can be influenced by controlling the conditions under which the compound is obtained in solid form. Solid state physical properties include, for example, the flowability of the milled solid. Flowability affects the ease with which the material is handled during processing into a pharmaceutical product. When particles of the powdered compound do not flow past each other easily, a formulation specialist must take that fact into account in developing a tablet or capsule formulation, which may necessitate the use of glidants such as colloidal silicon dioxide, talc, starch or tribasic calcium phosphate.
Another important solid state property of a pharmaceutical compound is its rate of dissolution in aqueous fluid. The rate of dissolution of an active ingredient in a patient's stomach fluid can have therapeutic consequences since it imposes an upper limit on the rate at which an orally-administered active ingredient can reach the patient's bloodstream. The rate of dissolution is also a consideration in formulating syrups, elixirs and other liquid medicaments. The solid state form of a compound may also affect its behavior on compaction and its storage stability.
These practical physical characteristics are influenced by the conformation and orientation of molecules in the unit cell, which defines a particular polymorphic form of a substance. The polymorphic form may give rise to thermal behavior different from that of the amorphous material or another polymorphic form. Thermal behavior is measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) and can be used to distinguish some polymorphic forms from others. A particular polymorphic form may also give rise to distinct spectroscopic properties that may be detectable by powder X-ray crystallography, solid state 13C NMR spectrometry or infrared spectrometry.
One of the most important physical properties of a pharmaceutical compound, which can form polymorphs or solvates, is its solubility in aqueous solution, particularly the solubility in gastric juices of a patient. Other important properties relate to the ease of processing the form into pharmaceutical dosages, as the tendency of a powdered or granulated form to flow and the surface properties determine whether crystals of the form will adhere to each other when compacted into a tablet.
The discovery of new solid states of a pharmaceutically useful compound provides a new opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic.